4/18/13 What Happened?

[Matt Hunt]

We started the day with a MDT risk, including a 10% hatched tornado area from IN all the way down to AR/MS. A PDS tornado watch was issued for IN at 9:30 am. As of 10:15 pm there are 3 tornado reports, and they all appear to have been brief, weak tornadoes. None of these were in the PDS watch area.

I'm not a complete beginner, but I'm also not an expert, so it makes me wonder what exactly happened today? I'd love to hear some meteorologists chime in. Were the models misleading? Was the forecaster a little too anxious?

From what I understand, a PDS watch is only issued when the forecaster has high confidence in multiple strong or violent tornadoes in the watch area. Given the time this watch was issued, I didn't see that within the watch area. If it was there at all, it wasn't until after that watch was set to expire (5 pm). This morning's RAP model had some large, curved hodograph's in the Wabash river vicinity and south in the late afternoon/evening hours. It was also showing near 2000 CAPE, if I remember correctly. I have seen a tendency for the RAP to overestimate CAPE. It seemed that the SPC expected these higher CAPE values to happen, and they seemed to believe they would happen by late morning. That's about all I can put together from today, that they expected CAPE to increase and discrete cells to fire ahead of the line, and/or the line to begin breaking into discrete, or at least embedded supercells. Still, in my mind that was a conditional threat, and did not warrant the PDS wording.

What do you think?

 

[Jeff Duda]

I have not had time to go over the post-storm wx data, but I think there were 2 or more serious problems. The storms in central and southwestern Oklahoma seeded one another, e.g. rain cooled outflow, and the cells had little or no right (eastward) turning, I recall. It will be interesting to look over the upper air data, especially with only 25 mph cell movements. I do not believe there was enough forcing along the dryline (where I was stationed near Seymour, TX). Another thing -- initial cell structure looked great on radar as they were developing, but pesky separate cells continued to form nearby. I believe a weak disturbance came through around 2-3PM (?) so there may have been some subsidence issues after the disturbance passed.

W.

I believe Matt is referring to Thursday's event.

Matt I have not looked too strongly at things either, but here's my overall take on the situation.

The PDS tornado watch could not have been well justified. Shortly after it was issued I looked at mesoanalyses and saw generally 1000 CAPE or less across the watch area (and even less outside the watch area). Although shear was pretty high, the area was becoming shrouded in clouds as the squall line was moving in from the west. Despite great moisture, I think there was just no instability for storms to work with, which is probably part of the reason why the squall line dissipated as it moved east. Regarding the risk outlook, risk outlooks tend to have a lot of momentum. Once a higher category risk is premiered, it takes a lot of lost confidence and poorer looking forecasts for severe storms for the risk category to be dropped. It was dropped at the 20Z update. Although the MDT designation was always for wind, never for tornadoes.

In general, probably hype from the previous day and overnight model forecasts (which probably turned out to not verify) were enough to "scare" the SPC forecasters into staying with a higher category risk. It seems in this business it is better to overhype and be wrong by going too high than to underhype and have people killed because you didn't have enough alert or warning beforehand. POD >> FAR. I know we have SPC forecasters on the forum, though, so perhaps one of them could shed some brighter light.

 

[Dave Kaplow]

I saw the PDS, looked at the parameters in that area, and immediately wondered if someone had made a mistake, especially since the actual wording of the watch didn‘t seem to me to be all that strong, not the kind of thing I’d expect to see in a typical PDS text. And sure enough, when I looked again a little later the PDS seemed to have vanished, so I just assumed the PDS part of the watch was issued in error. Do we know for sure that this was not the case? In retrospect it certainly does seem hard to justify a PDS for that area given the way the event was unfolding at the time, so I’d kind of like to think I’m right and it was just a simple mistake.

 

[chrisbray]

Despite great moisture, I think there was just no instability for storms to work with, which is probably part of the reason why the squall line dissipated as it moved east.

If this is too noob, you can move it to the introductory forum, but was this because of no time for surface heating due to early hours and/or the advancing line's cloud cover?

 

[Robert S. Thompson]

Insights from someone here in Indiana...

Somehow someone got the idea that there would be discrete supercells ahead of the main line, and I believe that prompted the hatched 10% and PDS (and it was an 7.5 hr watch!). I saw no evidence that it was a mistake, but they were quick to cancel it. I was surprised by PDS -- while models forecasted plenty of shear, CAPEs were marginal at best and models indicated that outflow and cloud debris from the dying line over IL would infiltrate IN. This time of year we cannot overcome cloudiness or recharge quickly. Also, looking at Skew-Ts revealed "long skinny" CAPES with a lack of cooling at mid and upper levels. And the shear was mostly speed shear, good for wind events but not tornadic supercells.

I did think a later squall line would blossom. But the winds never materialized, moisture and instability remained insufficient, and cold outflow from the north undercut everything.

I've observed some other (smaller) busts like yesterday in Indiana the past few Aprils. I think some forecasters see lots of shear and forget that the three ingredients for deep moist convection are instability, moisture, and a trigger. It is very rare to get sufficient instability and moisture north of the Ohio River before April and north of IND before May. Any cloud cover pretty much does us in unless the dynamics are impressive, which they weren't yesterday.

Also, the composite indices are heavily weighted toward shear. I think the SPC forecasters know better than too rely to heavily on composite indices, that reminder is for the rest of us. Yesterday was also a good reminder to look at observations and Skew-Ts. Too many people focus on CAPE, indices, and the (mostly) model-derived data shown on the mesoanalysis pages.

 

[Skip Talbot]

SPC seems to favor highly sheared setups, issuing higher probabilities for them. A 100 knot mid level jet overtaking a moist warm sector often results in a moderate or high risk. Also keep in mind that it doesn't take much CAPE at all when you've got the dynamic forcing from such a system. The Henryville, IN EF4 last March formed in an environment with what was about 1250-1000 of MLCAPE I believe and also produced baseball sized hail.

We had a 100 knot jet yesterday:

Most of the energy, however, was well behind the cold front, and the flow was also paralleling the cold front:

As a result there wasn't much lift ahead of the line and you wind up with a training mess stuck to the cold front. Helicity values were still quite high out across the warm sector, and with 1000 CAPE you can still get embedded supercells with strong tornadoes and discrete supercells just ahead of the line if there is a lobe of energy ahead of the jet, embedded shortwave, or some sort of boundary hiding in there. You're more likely to see discrete cells ahead of the front when that jet bisects the front rather than paralleling it, however. I was a little surprised to see the PDS seeing how much of a mess the cold front was and with the shear being displaced to the west, but these types of setups (low cape, high shear, cold fronts) are often what produce the OH river valley's violent tornadoes. Another busted forecast, but perhaps some of the parameters and pattern recognition in the region tripped the alarms, in addition to the difficulties in downgrading the threat probs, which requires consensus from all SPC forecasters, as noted above.

 

[chrisbray]

Thanks skip for that insightful post. It did seem like the squall line was well ahead of the front, and there wasn't much to initiate any cells ahead of it.

 

[Matt Hunt]

Thanks everyone for the replies! Busts like yesterday make me realize I still have quite a bit to learn!

 

[rdale]

The PDS was issued intentionally. I was shocked that it was issued as a PDS let alone a tornado. Not as shocked as I was to see the TOR for Michigan which was ridiculous. And then the evening tornado box for Indiana sealed the deal.

Learn the names of the issuing forecasters and it'll help you in future events

 

[Rob H]

SPC seems to favor highly sheared setups, issuing higher probabilities for them.

This is a good point, and one I looked into recently - considering Minnesota/Wisconsin seem to always get Moderate risks for extreme shear setups that bust. You have to remember that a risk has a probability, and that probability is always less than 100%. Every once in awhile, a high shear setup will do something like the 1992 Chandler, MN F5. So they don't always happen, but when they do:

http://www.crh.noaa.gov/images/fsd/events/chandler1992/chandler1.jpg

I'm curious as to how the SPC forecasts pre-frontal initiation. I still haven't seen many good resources discussing this, and I sometimes feel like they're just looking at a magic 8-ball.

 

[Jared Farrer]

We started the day with a MDT risk, including a 10% hatched tornado area from IN all the way down to AR/MS. A PDS tornado watch was issued for IN at 9:30 am. As of 10:15 pm there are 3 tornado reports, and they all appear to have been brief, weak tornadoes. None of these were in the PDS watch area.

I'm not a complete beginner, but I'm also not an expert, so it makes me wonder what exactly happened today? I'd love to hear some meteorologists chime in. Were the models misleading? Was the forecaster a little too anxious?

From what I understand, a PDS watch is only issued when the forecaster has high confidence in multiple strong or violent tornadoes in the watch area. Given the time this watch was issued, I didn't see that within the watch area. If it was there at all, it wasn't until after that watch was set to expire (5 pm). This morning's RAP model had some large, curved hodograph's in the Wabash river vicinity and south in the late afternoon/evening hours. It was also showing near 2000 CAPE, if I remember correctly. I have seen a tendency for the RAP to overestimate CAPE. It seemed that the SPC expected these higher CAPE values to happen, and they seemed to believe they would happen by late morning. That's about all I can put together from today, that they expected CAPE to increase and discrete cells to fire ahead of the line, and/or the line to begin breaking into discrete, or at least embedded supercells. Still, in my mind that was a conditional threat, and did not warrant the PDS wording.

What do you think?

I think everyone pretty much nailed what was wrong with the setup! I just wanted to add my opinion as well. "Instability just wasn't there"!!! But Skips right most people look at the shear and just assume its going to be noteworthy given theirs moisture. I honestly wouldn't have even issued a severe watch. Except maybe for rainfall potential. Most of west central Indiana has seen severe flooding which is rare. Well of this magnitude relatively speaking. But that is about the only outcome from yesterday! Storms did not fire out ahead of the line mostly because their was no instability. The lines that did move through were dynamically forced and completely outflow oriented. Little if any rotation was observed "tornado warnings". As far as I know hail was not observed. The line didn't even produce much in the way of winds. In fact most of Indiana is void of severe reports which is funny given the PDS watch. So quick and simple answer is Instability.

 

[STexan]

We're all identifying the missing ingredients come "go time" but I've still yet to see why the models have been as wrong as they have been this entire season practically in the days and hours ahead of system builds. I know forecasting and models are not an exact science but they generally improve with time. But this year, they (models) seem to be exaggerating systems on an almost predictable basis. I'm a photographer and enjoy storms. I'm not a professional weather forecaster and I don't presume to have a handle on all of what goes into storm prediction beyond the basics but am I the only one thinking something has changed between 2012 and 2013? Maybe it's the models weighting, maybe it's key models themselves having changed, maybe it's interpretation of the models, maybe it's a real-world weather pattern anomaly that is skewing everything ... I don't know, but I hope someone figures it out before city/county managers stop listening to the SPC. Rant over

 

[Dan Robinson]

I too was puzzled by the PDS, but not enough to avoid letting it sucker me into chasing the setup. I originally wasn't planning to chase at all after my morning data check. Here is what the squall line looked like at Macedonia, IL, just east of I-57 - it was tornado-warned at this time:



There were some weak couplets apparent on radar from this segment of the line all the way from Missouri to here, but visually the line didn't present any indications of even being close to producing. High cloud bases above outflow as far as you could see. KPAH radar was down close to this time, so I'm not sure if that contributed to the warnings being issued to play it safe. I'm definitely not going to bash the SPC on this, but I would be curious to hear what triggered the PDS. Everything's an opportunity to learn something.

Southern Indiana had clear skies in advance of the line. With the line stalling like that, the region to the east getting sun was going to have an opportunity to destabilize without the squall line plowing through early like we usually see them do in these setups. That may explain some of the concern for supercells ahead of the line.

Chase log:
http://stormhighway.com/blog2013/april1913a.shtml

 

[rdale]

We're all identifying the missing ingredients come "go time" but I've still yet to see why the models have been as wrong as they have been this entire season practically in the days and hours ahead of system builds.

I'm not sure I understand your premise... The models never said this would be a big day. When SPC went moderate with their DY2 I refused to use that term in my forecasts and posts because I disagreed.

I hope someone figures it out before city/county managers stop listening to the SPC.

Trust me, that will never happen. And remember the SPC is not one forecaster.

 

[STexan]

Maybe, but virtually all of the mainstream media outlets and the weather Channel follow the lead of the SPC. When the SPC goes moderate or High, the hype on all the cable news outlets gets ratcheted up. The general public is not privvy to all of the countering [to SPC forecasts] chatter that goes on by very knowledgeable people such as here on Stormtrack and others.